This invention concerns tufting machines, and more particularly to the feeding of yarn to the needles of such machines.
In the manufacture of tufted fabrics by a tufting machine having a multiplicity of reciprocable needles and a corresponding multiplicity of hooks or loopers selectively cooperable with respective ones of the needles, the needles and hooks or loopers being provided on opposite sides of a backing fabric and the needles penetrating the backing material to cooperate with respective hooks or loopers in forming loops at the reverse of such fabric, it is known to shift the needles laterally for patterning or other purposes, the needles, in such circumstances, moving to a position for cooperation with a different respective hook or looper from those with which the needles previously cooperated.
In the absence of lateral needle shift, each needle introduces a line of tufts which extends in the longitudinal direction of the backing material, the back stitch which exists at the needle side of the backing material between successive needle penetrations, and thus between successive yarn tufts, defining a sensible continuous straight line parallel to the direction of fabric movement.
If a lateral shift of the needle occurs between successive needle penetrations, the resultant back stitch will be of a zig-zag form and the back stitch extends in a direction which is diagonally inclined to the direction of fabric movement, and since the rate of advance of the backing fabric remains constant, the length of the individual back stitch will be increased accordingly. Thus, a greater length of yarn is required to form a fixed pile height loop. If the rate of yarn fed for that machine cycle is not increased, the yarn will be back robbed or withdrawn from the previously formed loop.
The yarn feed system of a tufting machine is traditionally by means of constant feed rate rollers wherein equal angles of rotation of a machine mainshaft produce equal angles of rotation of the yarn feed rollers. The rollers have a relatively high friction surface of rubber or the like, or a roughened or gritty material, such as sandpaper or the like, and the yarn is trained about the rollers and positively fed at a constant rate. The feed rate is sufficient to provide a loop for seizure by the hook or looper and to provide the back stitch between successive loops, and if there is a shortage of yarn for seizure by the needle, the yarn is back robbed from the previous stitch.
Such a direct drive feed roller system can not accommodate the changes in yarn feed requirement arising from lateral needle shift and gives rise to an uneveness in the pile height which detracts from the appearance of the fabric. In the case of cut pile fabrics, excessive shearing of the surface to level the pile height is required. A further consequence is that, due to varying yarn tension, difficulty may be experienced in attaining accurate alignment between the needle and the hook or looper and satisfactory loop seizure.